1. Field of the Invention
The present invention relates to a display apparatus that displays a color image by controlling the number of emissions or the intensity thereof in accordance with a plurality of primary color video signals input to it, and more particularly to a technique for correcting white balance in a plasma display apparatus that displays a color image by controlling the number of emissions of phosphors of three primary colors, red, green, and blue.
2. Description of the Related Art
Recently, research and development of various types of display apparatus has been proceeding; among them, the plasma display panel (PDP) has been attracting attention as a large screen flat display apparatus capable of crisply displaying characters, images, etc.
The plasma display panel achieves a color display by exciting phosphors of three primary colors, red, green, and blue and, in order to limit power consumption, for example, it is practiced to control the number of emissions (the number of sustain emissions) in accordance with image display ratio (Average Picture Level—APL). However, the luminance ratio among the respective color phosphors varies with the number of emissions; therefore, even when white balance is adjusted, for example, at a specified number of emissions, if the number of emissions changes, the white balance is shifted.
This white balance shift problem occurs due to changes in the number of emissions or the intensity of emission, not only in plasma display panels but also in various other display apparatuses such as display apparatuses using EL elements (electroluminescent elements), FEDs (field emission displays), LED (light emitting diode) displays, and CRTs (cathode ray tubes). Therefore, in a display apparatus that displays a color image by controlling the number of emissions or the intensity thereof in accordance with a plurality of primary color video signals input to it, it is necessary to maintain correct white balance regardless of the number of emissions or the intensity of emission.
Namely, phosphors of the three primary colors, red, green, and blue saturate in luminance as the number of emissions increases. This is because the persistence characteristics of the red, green, and blue phosphors, in other words, the energy conversion efficiency of the phosphors for excitation by ultraviolet radiation, decreases as the number of emissions increases. If white balance is adjusted at a specific point (A) where the number of emissions is large, the white balance value at that time is determined based on the luminance ratio among red, green, and blue at the specific point. On the other hand, when displaying an image in accordance with high APL video signals, the number of emissions is reduced in order to hold the power consumption within a predetermined value.
Accordingly, at another point (B) where the number of emissions is small, the energy conversion efficiency of the phosphors for excitation by ultraviolet radiation increases. If the rate of decrease of the energy conversion efficiency increases in the order of green, red, and blue, then the luminance increases relative to that at the specific point, in the order of green, red, and blue. That is, there is a difference in white balance between the specific point (A) and another point (B) because the luminance ratio among red, green, and blue at the other point (B) differs from the value used for adjustment at the specific point (A).
Conversely, when displaying an image in accordance with video signals whose APL is lower than that when the white balance was adjusted, the number of emissions may be increased, resulting in a further decrease in the energy conversion efficiency, and causing a difference in white balance because the luminance ratio among red, green, and blue changes, as in the case where the number of emissions is decreased.
The prior art and the problem associated with the prior art will be described in detail later with reference to accompanying drawings.
Though the present invention can be applied not only to plasma display apparatuses but also to various other display apparatuses such as display apparatuses using EL elements, FEDs, and CRTs, the following description will be given by dealing primarily with a plasma display apparatus as an example of a display apparatus that uses phosphors of three primary colors, red, green, and blue, whose persistence characteristics differ from one another.